In electrophotography, copied images are formed by various kinds of methods. For example, the surface of a photosensitive member is electrically charged and irradiated to form electrostatic latent images thereon, the electrostatic latent images are developed by a developer to be made visible and then the developed electrostatic latent images are fixed directly onto the photosensitive member (referred to as a direct method). In other method, developed electrostatic latent images on a photosensitive member which are made visible by a developer are transferred to copy paper and then, the transferred images are fixed on the paper (referred to as a powder transferring method). In another method, electrostatic latent images on a photosensitive member are transferred onto copy paper, the transferred electrostatic latent images are developed by a developer and then fixed on the copy paper (referred to as an electrostatic latent image transferring method).
Conventionally known photosensitive materials for forming a photosensitive member include inorganic photoconductive materials such as selenium, cadmium sulfide or zinc oxide.
These photoconductive materials have many advantages such as chargeability to an adequate potential level in the dark, low loss of charges in the dark, an electrical charge which can be rapidly dissipated with irradiation of light and the like. However, they have disadvantages. For example, a photosensitive member based on selenium is difficult to produce, has high production costs and is difficult to handle due to inadequate resistivity to heat or mechanical impact. A photosensitive member based on cadmium sulfide has defects such as its unstable sensitivity in a highly humid environment and loss of stability with the time because of the deterioration of dyestuffs, added as a sensitizer, by corona charge and fading with exposure.
Many kinds of organic photoconductive materials such as polyvinylcarbazole and so on have been proposed. These organic photoconductive materials have superior film forming properties, are light in weight, etc., but inferior in sensitivity, durability and environmental stability compared to the aforementioned inorganic photoconductive materials.
Various studies and developments have been in progress to overcome the above noted defects and problems. A function-divided photosensitive member has been proposed, in which charge generating function and charge transporting function are divided to form a photosensitive layer on an electrically conductive substrate (for example aluminum). Such function-divided photosensitive members have high productivity and low costs since they can be prepared by coating, and suitably selected charge generating materials can freely control a region of photosensitive wavelength.
However, when a photosensitive member above mentioned is used repeatedly, there arise such problems as decrease of initial surface potential, gradual increase of residual potential and formation of fogs in copy images. These problems may be brought about by interface conditions between a charge generating material and a binder resin or charge transporting material and binder resin, energy barrier, impurities, corona-discharge, image-irradiation, deterioration of materials caused by irase-lamp, adsorption of oxidizing gas such as ozone, NOx etc., deterioration of materials caused thereby. Therefore, many trapping positions generate in a photosensitive layer.
Generated charges may be caught at the trapping positions before encountering surface charges.
In order to prevent the increase of residual potential etc., there are proposed many techniques such as removal of impurities from materials, prevention of composition from deterioration by addition of antioxidant (for example Japanese Patent Laid-Open Sho 57-122444), addition of electronattracting compounds (for example Japanese Patent Laid-Open Sho 58-7643, Japanese Patent Laid-Open Sho 58-54346). However, the fact is that a photosensitive member excellent in long repetition can not be obtained.
Further, higher reliability on copy images and repetition stability are required than before because a photosensitive member is also applied to a laser-printer.